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  DB<200> p
bab
...
  DB<200> x / (.) (?! .* \1 ) /gx
0  'a'
1  'b'

  DB<211> x / (.) (?<! \1 .* ) /gx
Variable length lookbehind not implemented in regex m/ (.) (?<! \1 .* 
+) / at (eval 261)[C:/Perl_64/lib/perl5db.pl:646] l
ine 2.